No exclusions. Arguably not a true 20% off as the 20% is converted into non-refundable 'store credit'. But as a regular shopper this will not be an issue.
Maximum off is $20
Example: Buy GTA V for $70 and receive $14 worth of player points.
Annoyingly, (I'm assuming) the process for redeeming player points into usable vouchers is in tiers eg $5, $10, $15 etc so getting $14 of points will only allow for a $10 voucher unless you have some points already or earn some in the future.
That's not OFF at all, there is no discount in this deal.